An Intra-Individual Comparison of MRI, [(18)F]-FET and [(18)F]-FLT PET in Patients with High-Grade Gliomas

OBJECTIVES: Intra-individual spatial overlap analysis of tumor volumes assessed by MRI, the amino acid PET tracer [(18)F]-FET and the nucleoside PET tracer [(18)F]-FLT in high-grade gliomas (HGG). METHODS: MRI, [(18)F]-FET and [(18)F]-FLT PET data sets were retrospectively analyzed in 23 HGG patients. Morphologic tumor volumes on MRI (post-contrast T1 (cT1) and T2 images) were calculated using a semi-automatic image segmentation method. Metabolic tumor volumes for [(18)F]-FET and [(18)F]-FLT PETs were determined by image segmentation using a threshold-based volume of interest analysis. After co-registration with MRI the morphologic and metabolic tumor volumes were compared on an intra-individual basis in order to estimate spatial overlaps using the Spearman's rank correlation coefficient and the Mann-Whitney U test. RESULTS: [(18)F]-FLT uptake was negative in tumors with no or only moderate contrast enhancement on MRI, detecting only 21 of 23 (91%) HGG. In addition, [(18)F]-FLT uptake was mainly restricted to cT1 tumor areas on MRI and [(18)F]-FLT volumes strongly correlated with cT1 volumes (r = 0.841, p<0.001). In contrast, [(18)F]-FET PET detected 22 of 23 (96%) HGG. [(18)F]-FET uptake beyond areas of cT1 was found in 61% of cases and [(18)F]-FET volumes showed only a moderate correlation with cT1 volumes (r = 0.573, p<0.001). Metabolic tumor volumes beyond cT1 tumor areas were significantly larger for [(18)F]-FET compared to [(18)F]-FLT tracer uptake (8.3 vs. 2.7 cm(3), p<0.001). CONCLUSION: In HGG [(18)F]-FET but not [(18)F]-FLT PET was able to detect metabolic active tumor tissue beyond contrast enhancing tumor on MRI. In contrast to [(18)F]-FET, blood-brain barrier breakdown seems to be a prerequisite for [(18)F]-FLT tracer uptake.